A wide variety of methods and apparatus are known for inspecting and measuring gears. Determination of variance or error in gears has become a highly refined technology which utilizes very complex electronic, optical, and mechanical methods. As a result, most gears are currently inspected in a sophisticated testing machine. Such machines require substantial capital investment (most machines, in 1991, sell in the range of $75,000 to $175,000), and are generally operated by highly skilled workmen. Quite simply, the sophisticated machinery currently employed for gear inspection is too expensive for many companies to own or lease. Consequently, testing of gears is minimized in order to reduce manufacturing costs. Instead, process control is emphasized, hoping to achieve gear accuracy by control of manufacturing equipment, methods, and processes, without resort to inspection of individual elements of every gear or of every few gears produced. Often, only an initial gear from a production run is closely inspected by modern gear testing apparatus. Subsequent gears, theoretically cut to the same specifications, are often shipped to end users of the product without rigorous inspection of the tolerances. Inspection rates frequently run as low as 1 to 5%. Even when gear inspection is conducted by machine, work must often be shipped to a specialized gear lab, which may take up to several weeks to provide results, at costs running $130 per hour of inspection, or more. Thus, only statistically selected gears from a large production run may be inspected for critical parameters such as the accuracy of the helix on a helical gear. Obviously, the current practice of limited gear inspection greatly increases the chance that improperly fitting or operating gears may be sent out from the manufacturing shop.
The gear inspection apparatus and method of the present invention may be utilized with a variety of gear types, including helical gears, bevel gears, spiral bevel gears, worm gears, or other gear forms where testing may be conducted by utilizing complimentary master gear segments, gear segment holders or support blocks, and measuring devices and indicators as taught herein. However, since this invention is likely to be most often utilized in conjunction with the inspection of helical type gears, some familiarity with the manufacturing processes for such gears will assist the reader in understanding the invention as set forth below.
A helical gear is one in which the individual teeth follow a helix, i.e., surround an imaginary cylinder for some gear face length along the gear axis. If the surface of that cylinder was unrolled and flattened out, the helical gear teeth would form a series of parallel straight lines. The angle between those straight lines and the axis of the gear is the helical angle of the gear.
Helical gears are identified as right-hand or left-hand, depending upon the direction that the helix slopes away from the viewer, when the line of sight is parallel to the axis of the gear. This is important since in order for two helical gears on parallel shafts to mesh, they must be of different hands. As further described below for the present invention, if the measured gear is right hand, then the selected master gear segment utilized to inspect that measured gear must be left hand. Also, it will be recognized that the helix angles of right hand and left hand gears on parallel shafts must be equal.
Helical gears are most often manufactured by utilizing a rotating cutting machine known as a hob or hobbing machine. The hob is actually the cutting tool utilized in the process; the process is accomplished by moving the hob across the gear blank as both the gear blank and the hob are rotated. When helical teeth are cut, the advance of the hob carriage in relationship to the rotation of the gear blank can be adjusted to produce the proper helix angle. Thus, by selection of the proper gear ratio for the hobbing machine drive, the hob is advanced in correct relation to the gear blank so as to cut the specified number of teeth at the correct helical angle. It is important that a gear blank be securely held and that the gear blank be turned precisely at the desired ratio between the advance of the cutter and the axis of the gear. For instance, helical gears mounted on an arbor are more likely to slip when being cut than is a spur gear when it is being cut, because the pressure of the helical cut, being at an angle rather than straight across the gear blank in the direction of the axis (and thus the arbor) as in the case of a spur gear, will tend to rotate the helical gear blank on the arbor. When such slippage occurs, an error in helical angle may result. This error is known as helical angle deviation.
The just described problem which is encountered in the manufacturing of helical gears may result in helical angle deviation for all teeth for a portion across the gear face; in that portion, all teeth would be circumferentially displaced. Also, there may occur a partial dislocation of the entire tooth form. Helical angle deviation is by far the major problem commonly encountered in machining gears.
Helical angle deviation may also be caused by wobble during manufacture, i.e., the gear blank may have been improperly mounted on the gear manufacturing machine, with the result that teeth on portions of the blank were cut at other than the desired helix angles. In addition, helical angle deviation may be caused by improper selection or operation of various gears in the gear cutting machine.
These different kinds of manufacturing problems might result in different kinds of gear defects. For instance, one gear might be manufactured to the wrong helical angle, i.e., it is perfectly made from the standpoint that helical angle is uniform throughout the entire gear, but the actual helical angle is not within the desired specifications. Another problem might be that the gear is satisfactory for most of the revolution but has a portion of the face or a section of the circumference where the helix angle deviates beyond the allowable specification for the particular class of gears. In any event, it is important to discover gear defects as early as possible in the manufacturing process, to reduce costs of rejected parts or from field failure of improperly operating gears.
From the foregoing, it is clear that there is a continuing need for a simple, low cost, quickly executable gear inspection method and for the apparatus to enable semi-skilled or unskilled personnel to confidently, accurately, and reliably carry out the inspection method.